Injection heroin use and HIV/AIDS are major public health problems in Vietnam, with HIV infection continuing to be concentrated among the growing injection drug use (IDU) population (20% HIV prevalence, nationally). Although considerable progress has been made to expand HIV prevention and antiretroviral therapy (ART) programs, the high HIV transmission rate among IDUs continues (HIV incidence, 5% per year), access to antiretroviral therapy still limited (11%), and mortality among HIV-positive drug users remains excessive (15% per year), requiring interventions. The Vietnamese government's approach to drug demand reduction has primarily relied on its system of 83 drug treatment centers (DTC) where most detainees (85%) are admitted through arrests by police. Relapse to opioid injection after such treatment is 71% within 6 months after release from DTCs. DTCs represent a unique opportunity to identify, test, and provide ART to a large segment of this marginalized population. We propose an intervention that employs the seek, test, and treat strategies for the IDU population and their network members, by ensuring that high risk individuals are sought for HIV testing, promptly referred to and maintained on ART, while HIV prevention interventions provided to the many who test HIV-negative, as well as those who test HIV-positive. The intervention will seek and test IDUs via DTCs using CDC-recommended routine opt-out HIV testing in criminal justice settings and also will seek and test their network members. HIV-positive IDUs will be referred to HIV care through a two arm randomized controlled trial to compare the effects of different levels of engagement in care on ART uptake, antiretroviral adherence, and treatment outcome. The total duration of the trial will be five years. The study will be conducted in all participating DTCs in the capital city of Hanoi